The present invention relates to measurement instruments, and in particular to accelerometers.
An accelerometer measures the rate at which the velocity of an object is changing. A typical accelerometer utilizes a proof mass, a spring (or its equivalent) joining the proof mass to a stationary housing or substrate, and a sensor to measure displacement of the proof mass. The accelerometer is attached to the moving object, and as the object accelerates, inertia causes the proof mass to lag behind as its housing accelerates with the object. The force exerted on the proof mass (given by Newton""s second law) is balanced by the spring, and because the displacement allowed by the spring is itself proportional to applied force, the acceleration of the object is proportional to the displacement of the proof mass.
The displacement sensor is the key component in determining overall accelerometer performance in terms of sensitivity, stability, and packaging constraints. Presently available devices utilize any of several sensing techniques, including capacitative, piezoresistive, piezoelectric, and tunneling approaches. The most sensitive accelerometers use a tunneling sensor, which can measure displacements as small as 10xe2x88x924 Angstrom per root hertz. See, e.g., Liu et al., J. Microelectromech. Sys. 7:235 (1998).
Tunneling detection, however, is inherently nonlinear and requires the sensing electrodes to be in close proximityxe2x80x94typically 10 xc3x85. Moreover, the tunneling sensitivity is strongly sensitive to electrode contamination from the ambient environment. To address these requirements, it is typically necessary to employ high-voltage feedback circuitry to control the position of the proof mass electrode relative to the tunneling electrode. The fabrication process that integrates electrostatic actuators for controling the proof mass is also complex, involving several photolithographic masks and wafer bonding.
The present invention utilizes optical interference to measure displacement of the proof mass (and, consequently, acceleration of the moving object under study). This approach combines the reliability of optical devices, which are not affected by ambient electrical conditions, with sensitivities comparable to those of tunneling accelerometers. The invention is well-suited to fabrication of arrays that improve resolution through differential measurements.
A preferred embodiment utilizes a proof mass having a first set of spaced-apart, elongated fingers projecting therefrom, and a stationary housing or substrate comprising a second set of similarly arranged projecting fingers. A spring connects the proof mass to the substrate such that, in a rest configuration, the first and second set of fingers interdigitate. When the structure is accelerated, the substrate fingers remain stationary, while the alternating fingers of the proof mass are displaced. This creates a phase-sensitive diffraction grating which, when illuminated, facilitates determination of the relative displacement between the sets of fingers by measuring the intensity of the diffracted modes. This displacement, in turn, indicates the acceleration experienced by the accelerometer structure.
The illumination source and detector can be fabricated as a unit, or surface mounted on a circuit board such that they can be aligned within close proximity to the accelerometer.